CN1656339A

CN1656339A - Control of cyclone burner

Info

Publication number: CN1656339A
Application number: CNA038121077A
Authority: CN
Inventors: B·永达尔
Original assignee: TPS Termiska Processer AB
Current assignee: TPS Termiska Processer AB
Priority date: 2002-05-29
Filing date: 2003-05-21
Publication date: 2005-08-17
Anticipated expiration: 2023-05-21
Also published as: BR0311340A; CA2487335A1; EP1532393B1; JP4181117B2; DE60322227D1; UA79967C2; CN1320305C; HK1081637A1; US20050132942A1; RU2315907C2; SE522006C2; NO326381B1; SE0201621L; HRP20041067A2; RU2004138287A; AU2003232869A1; ATE401533T1; SE0201621D0; US7261047B2; NO20044956L

Abstract

A method of operating a combustion process in a cyclone burner, after start-up thereof, is provided. A fuel and a combustion gas is fed into a combustion chamber of the cyclone burner. The velocity of the combustion gas is kept between a lower and an upper limiting gas velocity. The stoichiometric condition (sub- or over- stoichiometric) is maintained by controlling the amount of fed oxygen to the amount of fed fuel. A shift is made to the other stoichiometric condition while preventing the combustion gas from obtaining a velocity outside the range defined by the lower and upper limiting gas velocity.

Description

Control of a cyclone burner

Technical Field

The invention relates to a method for controlling a combustion process in a slag-free cyclone burner after starting the burner.

Background

The cyclonic preheat or hearth burner may be described as an "adiabatic" circulation burner having a combustion chamber into which combustion gases, such as air, are introduced tangentially to form a swirling flow. Fuel particles are introduced into the gas stream and by centrifugal forces acting on them they will be transported along the combustion chamber wall. The fuel in the cyclonic burner preferably comprises milled particles, but the requirements for refined materials are much lower compared to free-standing solid fuel burners.

In many applications, the temperature inside the cyclone burner is so high that the fuel ash melts and forms slag, which must be continuously removed from the burner. This is often the case when heating coal (firecoal) is used. In other applications (typically wood combustion), the temperature will be controlled to avoid the occurrence of molten ash (sticking).

In most applications, the cyclone burner is fitted with a refractory lining to prevent corrosion and minimize heat loss. In combination with the high heat density, this results in an approximately adiabatic temperature within the combustor.

In many applications it is desirable to keep the temperature within a certain temperature range in order to obtain a satisfactory carbon burn-off, while avoiding disadvantages such as the above-mentioned sticking in case of high temperatures. The highest temperature will be reached just below stoichiometric conditions, i.e. when the oxygen of the added combustion gas or air is equal to the amount of oxygen used for complete combustion of the fuel. The same is true if less oxygen is added, i.e. under sub-stoichiometric conditions, the temperature will be lower, and if more oxygen is added, i.e. over-stoichiometric conditions, since the excess oxygen will act as a cooling medium. This will be illustrated in figure 1.

Turndown ratios, i.e., the ratio of maximum to minimum operable fuel loads for a given cyclone burner, are limited (simplified) by particulate circulation requirements and large particulate carryover. In other words, the gas flow rate or gas velocity should be above a lower limit in order to avoid entrainment of fuel particles while not entraining them due to gravity and friction, and below an upper limit in order to avoid escape of particles from the combustion chamber before complete combustion.

Slagging cyclone burners are one of the most common applications. They operate under over-stoichiometric conditions, primarily to avoid a corrosive environment under reducing conditions when burning coal. In general, an adjustment ratio of about 2: 1 is possible. Slagging cyclone burners are used to completely melt ash particles (which are primarily taken as slag). In contrast, the slagging-free cyclone burner operates without severe slagging within the burner. Thus, the ash is primarily withdrawn as solid fly ash particles. Although sub-stoichiometric is the most common case, the slagging-free cyclone burner can be operated either under sub-stoichiometric conditions or under over-stoichiometric conditions. In general, a 4: 1 turndown ratio is possible. It is preferable to operate at sub-stoichiometric conditions because the burner can be built more compact. Specific gas volume flow (m) through a cyclone burner³/Kgfuel) can be seen as approximately proportional to the stoichiometric ratio, so higher thermal loads are possible at sub-stoichiometric conditions.

The prior art provides little controllability of the combustion process for the swirl burner and it is difficult to achieve turndown ratios of greater than 4: 1 when operating within the desired temperature range. The main reason for this is because at high gas flow rates the residence time of the fuel particles in the combustion chamber is limited, or because at low gas flow rates the circulation in the combustion chamber becomes insufficient. One possible solution to obtain a larger turndown ratio is to provide a longer burner. However, such a construction would be expensive, heavy and require a lot of space. In addition, longer burners present considerable layout difficulties if conventional existing burners are to be replaced.

Disclosure of Invention

It is an object of the present invention to provide a method enabling enhanced controllability and adjustability of a compact slag-free cyclone burner.

It is another object of the present invention to provide a method of increasing the turndown ratio possible for a given cyclone burner.

These and other objects that will be apparent from the following description are achieved by means of a method as defined in the appended claims.

The invention is based on the insight that by shifting between sub-stoichiometric and over-stoichiometric conditions in the same region of the combustion chamber of the slagging-free cyclone burner, an improved adjustability and a larger turndown ratio can be obtained compared to the prior art.

Generally, it is desirable to maintain the temperature within the combustion chamber of a swirl burner within a limited temperature range. The lower the temperature in the combustion chamber, the slower the burning rate of the obtained char particles (residues after pyrolysis) and thus the carbon build-up in the burner may also result in a lower output from the cyclone burner. Suitably, the lower limit of the temperature range is at least 700 ℃ and preferably 900 ℃. In some cases, however, such as for certain fuel materials, the limit may be even lower, such as 600 ℃. The upper limit of the temperature range depends inter alia on the melting and sticking of the combusted fuel. Suitably, the upper limit of the temperature range is up to 1300 ℃ and preferably 1100 ℃. However, in some cases, for example for certain fuel materials, the limit may even be higher, for example 1400 ℃. This means that the amount of combustion gas should be controlled in relation to the amount of fuel present in the combustion chamber in order to keep the temperature within a desired range. In other words, according to at least one embodiment of the present invention, by controlling the amount of oxygen fed relative to the amount of fuel fed, one of two stoichiometric conditions (sub-stoichiometric and over-stoichiometric) can be maintained.

Thus, if the load (i.e., the amount of fuel fed into the combustion chamber) is reduced, the combustion gas flow may also be reduced in order to maintain the same stoichiometric conditions. The lowest possible gas flow or gas velocity for maintaining circulation of the fuel particles will therefore normally set the lower limit of the load. We have recognised that if the swirl burner is operated at sub-stoichiometric conditions it is possible not only to reduce the load to the load limit (at which the gas flow will be at the edge of insufficient circulation), but also to reduce the load to even lower loads by shifting to over-stoichiometric conditions at the load limit. This means that an excessive amount of combustion gas is suddenly supplied, so that the load can be considerably reduced. Both sub-stoichiometric and over-stoichiometric conditions can maintain the temperature within a desired temperature range.

As mentioned previously, the operation of a cyclone burner is limited by the following factors: a) a minimum or lower gas velocity that ensures circulation of fuel particles, and b) a maximum or upper gas velocity set by a limit at which entrainment of unburned particles becomes too high. For a given cyclone furnace and a given fuel, it is possible to choose either to operate at a relatively low maximum load under over-stoichiometric conditions or to operate at a relatively high minimum load under sub-stoichiometric conditions. By combining the operating modes, the turndown ratio can be increased.

According to one aspect of the invention, a method of controlling a combustion process in a cyclone burner is provided. According to the method, fuel is fed into a cylindrical combustion chamber of a cyclone burner and an oxygen-containing combustion gas, such as air, is introduced into said combustion chamber with a tangential velocity component to provide at least part of the fuel circulation for the fuel to be vaporized or combusted along the combustion chamber walls. A lower limit gas velocity and an upper limit gas velocity are defined for the combustion gas. The velocity of the combustion gases is maintained between the limit gas velocities. By controlling the amount of oxygen delivered relative to the amount of fuel delivered, either sub-stoichiometric or over-stoichiometric conditions can be maintained within the combustion chamber. The method further includes transitioning to the other of the two stoichiometric conditions while preventing the combustion gases from attaining velocities outside of the range defined by the lower and upper limit gas velocities.

This means that the velocity of the combustion gas will not be lower than the lower limit gas velocity and not higher than the upper limit gas velocity, regardless of the direction of transfer, i.e. from sub-stoichiometric to over-stoichiometric conditions, or vice versa. This applies both before and after the activity of the transfer from one stoichiometric condition to another, and also during the actual transfer.

For a given temperature within the combustion chamber, the temperature, together with the limiting gas velocity, define a possible transition region, i.e., a fuel load range, for which a transition or shift from one to the other of two stoichiometric conditions is possible in accordance with the teachings of at least one embodiment of the present invention. The minimum and maximum fuel loads of the range are dependent on the temperature.

It has been found that the possible transition region can be extended by mixing the recirculated flue gas with the oxygen-containing combustion gas before feeding the combustion gas into the combustion chamber. In other words, for each given temperature, the addition of recirculated exhaust gas to the oxygen-containing combustion gas will result in a lower minimum fuel load (compared to the case where no recirculated exhaust gas is added).

The addition of recirculated exhaust gas affects both sub-stoichiometric and over-stoichiometric conditions. The turndown ratio at sub-stoichiometric conditions can be further extended if the recirculated exhaust gas is mixed with the combustion gases before they are provided to the combustion chamber. This effect is twofold. First, recirculating the exhaust gas increases the gas flow rate without increasing the heat released from the fuel. The stoichiometric ratio depends on the amount of oxygen-containing gas. Since some of this oxygen-containing gas may be replaced by essentially oxygen-free exhaust gas (or with a very small amount of oxygen), sub-stoichiometric conditions will be available for even lower loads than would be the case if no exhaust gas were recirculated, without compromising the effectiveness of the recirculation. Thus, the minimum limit of gas flow is reached at lower loads. Second, the recirculated exhaust gas acts as a ballast. Therefore, additional oxygen-containing gas, such as combustion air, is required in order to release more heat from the fuel, thereby maintaining the temperature, and in other words, the stoichiometric ratio is moved slightly closer to 1. This means that the minimum limit is reached at a further low load.

Under over-stoichiometric conditions, the added exhaust gas will partially replace the excess combustion air. The exhaust gas will act as a ballast, which means that the same amount of fuel will heat a larger mass, so that less combustion air can be used for cooling. In case the total gas flow remains the same, the advantage is that the oxygen concentration will decrease. Thus, less nitrogen oxide will be formed.

The main effect of using recirculated exhaust gas is an increased load range, in which operation at sub-stoichiometric conditions is possible.

As an alternative to recycling the exhaust gas, it would be possible to obtain similar results, i.e. to extend the possible transformation area, by mixing the combustion gas with any inert gas or gases containing a lower percentage of oxygen.

While it is possible to vary the amount of combustion gas (such as air) in order to control the temperature within the combustion chamber, an alternative approach is to use recirculated exhaust gas (or inert gas or low oxygen-containing gas) to control the temperature within the combustion chamber. This is advantageous when it is desired to maintain a predetermined stoichiometric ratio, wherein the temperature is controlled by varying the amount of recirculated gas added to the combustion gases. The gas velocity is maintained within predetermined limits.

According to at least one embodiment of the invention, the stoichiometric conditions may be controlled without having to mix any additional inert or recirculated exhaust gas with the combustion gases. In this case, by controlling the amount of combustion gas fed in accordance with the amount of fuel fed, a substantially constant stoichiometric ratio between oxygen and fuel, which is not equal to 1, can be maintained, i.e., in one of two states (sub-stoichiometric and over-stoichiometric). One substantially constant stoichiometric ratio is maintained prior to the transfer event and another stoichiometric ratio is maintained after the event of transferring from one stoichiometric condition to another. Thus, if a relatively low load is present, i.e. a low fuel quantity is fed to the combustion chamber, a substantially constant over-stoichiometry ratio can be maintained until a substantially constant sub-stoichiometry ratio is shifted, the shift time (among other things) depending on the magnitude of the load. The term substantially constant stoichiometric ratio is to be understood as allowing a change in the stoichiometric ratio that provides a temperature within a certain desired temperature range. For example, and by way of illustrative example only, reference is made to FIG. 1 wherein the (sub-) stoichiometric ratio should be about 0.4-0.45 and the (over-) stoichiometric ratio should be about 1.8-2 for a temperature range of 1200 deg.C-1300 deg.C. Thus, before and after the transfer time, but not during the transfer time, the amount of combustion gas will increase or decrease, respectively, as the load is increased or decreased, so as to maintain a substantially constant stoichiometric ratio.

There are different options for controlling the amount of combustion gas fed to the combustion chamber. The limiting factors are the lower and upper gas velocities in the combustion chamber. The velocity of the combustion gases provided from the combustion gas inlet will be substantially maintained as the gases enter and pass tangentially through the combustion chamber, i.e. losses can be considered negligible. In view of this, a straightforward design is to provide the combustion gas inlet with a fixed cross-sectional area. The velocity of the gas can be controlled by increasing or decreasing the amount of combustion gas entering the combustion chamber. Alternatively, the combustion gases may be selected to be supplied so as to achieve a fixed velocity (one level between the limit gas velocities) and instead vary the open area of the intake port. A large opening area is used when a large flow, i.e. large gas volume, is desired, and a small opening area is used when a small gas volume is desired. As already explained above, the desired amount of gas depends on the amount of fuel. A further alternative control method is to vary both the cross-sectional area of the inlet and the velocity of the supplied combustion gases. Thus, in all three cases, the gas flow rate, i.e. the volume per unit time, is controllable.

A velocity meter or flow meter may be provided in the gas supply duct for measuring and calculating the velocity of the combustion gas. Accordingly, a measuring device, such as a speedometer or flow meter, may be provided for calculating the amount of fuel being fed into the combustion chamber. Such measurements and calculations are suitable as a basis for determining the time to transition from one stoichiometric condition to another.

The method for controlling the combustion process in a cyclone burner is suitable for solid, liquid or gaseous fuels. It has been found to be particularly suitable for use with solid fuels. The solid fuel is suitably a biofuel of some kind. The solid fuel may be in the form of particles such as wood particles, preferably wood pellets, typically crushed wood pellets of maximum diameter 4 mm.

When solid fuel particles are used, the lowest velocity for keeping at least most of the fuel particles circulating in the combustion chamber is set to the lower limit gas velocity. The lower limit gas velocity may also be set based on the maximum particle size of the fuel or on some other basis. For example, certain types of fuel particles entering the combustion chamber will rapidly release their volatile species, thereby reducing the particle density. In this case, therefore, it is appropriate to determine the minimum or lower tangential gas velocity on the basis of the particle density obtained after devolatilization. For wood particles, the density is generally 250kg/m³About one quarter of the density of the particles before entering the combustion chamber.

For "horizontal" swirl burners, i.e. comprising a combustion chamber with a central axis of symmetry extending horizontally, the lower limit gas velocity is suitably set so as to meet certain criteria at the top of the combustion chamber.

For a cyclone burner combustor having a horizontal central axis and a circular cross-section in the vertical plane, it is believed that the circulating gas flow within the combustor is not expanded and therefore the tangential peripheral velocity is equal to the gas inlet velocity.

Five forces act on the fuel particles, namely:

gravity F_g＝-m_pg

Centrifugal force

F_{c} = m_{p} \frac{V_{p, t}^{2}}{R}

Frictional force F_f＝-μm_pα_N

Tangential traction force

Radial traction force

Wherein,

m_pmass of particles

g-gravity constant

Radius of combustion chamber of swirl burner

V_g，tTangential gas velocity

V_g，rRadial gas velocity

V_p，tTangential particle velocity

V_p，rRadial particle velocity

Mu-friction factor

α_NAcceleration in normal direction

C_dCoefficient of traction

A_pCross-sectional area of fuel particle

ρ_gDensity of combustion gas

In the case where the particle at the uppermost position (top) is just prevented from falling, the lower limit gas velocity may be appropriately set. This is the case when gravity and radial traction balance the centrifugal force, resulting in zero friction. The limiting tangential particle velocity becomes:

the radial drag can be assumed to be negligible and the limit tangential particle velocity (V)_p，t) Is represented as:

V_{p, t} = \sqrt{gR}

however, the tangential gas velocity in the combustion chamber must be greater than the limit particle velocity. The lower limit gas velocity, and hence the gas velocity, which ensures the desired particle velocity at the top of the cyclone burner, is obtained by solving the following differential equation.

Thus:

where Φ is the angle from the vertical, i.e., 180 at the top of the chamber, and S is the distance traveled by the particle along the circumference.

Given the desired top particle velocity

V_{p, t} = \sqrt{gR}

To solve the tangential gas velocity V_g，tIt can be found that V increases as the radius of the combustion chamber and the particle diameter of the cyclone burner increase_g，tAnd (4) increasing.

In "vertical" cyclone burners, i.e. combustion chambers with a vertically extending central axis of symmetry and a circular cross-section in the horizontal plane, the forces acting on the particles are similar to the forces to which the particles are subjected in "horizontal" cyclone burners with an additional vertical pulling force. However, for simplicity, both radial and perpendicular forces are considered negligible. With this assumption, solving the following equation (which will be discussed further in conjunction with FIG. 11) allows the tangential lower limit gas velocity V to be calculated_g，t：

Wherein

V_g，tTangential gas velocity

g-gravity constant

Radius of combustion chamber of swirl burner

Alpha is the angle to the horizontal

Mu-friction factor

d_pDiameter of fuel particles

ρ_pDensity of fuel particles

ρ_gDensity of combustion gas

C_dCoefficient of traction

Alternatively, the lower limit gas velocity may be determined empirically (i.e., tested against a particular cyclone burner combusting a particular fuel). The method according to the invention is applicable regardless of how the lower limit gas velocity is determined.

The upper limit gas velocity is suitably set to the highest velocity allowed in order to minimize the number of unburned fuel particles leaving the combustion chamber, said velocity being 20-50m/s, preferably 25-40m/s, such as about 30 m/s. Another definition of the upper limit gas velocity is 3-6 times, typically 4 times, the lower limit gas velocity.

It may be expected that the separation efficiency, i.e. the tendency of particles to move along the combustion chamber wall, will increase indefinitely as the tangential gas velocity increases. However, in practice, at a certain velocity, the entrainment of particles towards the central axis of the combustion chamber again becomes very pronounced due to the increased turbulence and vortex breakdown within the cylindrical combustion chamber of the cyclone burner. Although the upper limit gas velocity is not directly calculated, it is empirically understood that a typical value is about 30 m/s.

Another aspect that limits the possible upper gas velocity is the volumetric concentration of unburned fuel particles in the combustion chamber. It is the burn-out time of the char (the residue after fuel devolatilization) that plays a limiting role. For a given temperature and stoichiometry, the amount of unburned char within the combustion chamber of the cyclone burner will be proportional to the load, and thus also the tangential gas velocity. At a certain load, the concentration of unburned fuel particles will become so high that carry-over again will become quite noticeable. Under over-stoichiometric conditions, re-carryover due to high tangential velocity is likely to be the limiting factor. At sub-stoichiometric operation, re-entrainment due to fuel particle blockage is more likely to be the limiting factor.

The process for determining the upper limit gas velocity may vary, for example, by testing for a particular cyclone burner that burns a particular fuel. The method according to the invention is applicable regardless of how the upper or lower gas velocity is determined. They have the effect of limiting the value. For example, in accordance with at least one embodiment of the present invention, the act of transitioning from one of the two stoichiometric conditions to the other is performed just before the gas reaches one of the limit gas velocities. According to at least another embodiment of the invention said shift to the other of said two conditions is performed when such an amount of combustion gas (which for the other stoichiometric condition corresponds to a gas flow velocity within a limit gas velocity interval) would be required for the amount of fuel fed under the present stoichiometric conditions.

As discussed previously, the method according to the invention provides a much larger turndown ratio for the cyclone burner than was possible in the prior art. Nevertheless, it is desirable to maintain the temperature within an interval that is actually useful to further increase the turndown ratio for both sub-stoichiometric and over-stoichiometric conditions. Although a temperature range of 900-1100 c is preferred inside the cyclone burner, an extension to the range of 700-1300 c, and even larger ranges, is acceptable. For example, if a higher than normal temperature can be allowed during sub-stoichiometric conditions, such as near or about 1300 ℃, more oxygen is required than usual so that the temperature can be increased for the same amount of load. This means that the stoichiometric ratio is closer to 1, since more oxygen-containing gas relative to the load is allowed to be introduced to the cyclone burner, the result is that a lower minimum load is allowed while still introducing enough gas to keep the particles circulating. Similarly, during over-stoichiometric conditions, a relatively lower temperature, i.e., more oxygen relative to load, may be allowed. This may also result in a lower minimum load.

Even if it is possible to use different temperatures, in many cases it is desirable to maintain as uniform a temperature as possible. This applies in particular to the actual time of transition from the sub-stoichiometric ratio to the over-stoichiometric ratio and vice versa. Suitably, therefore, this transfer is performed rapidly in order to maintain the temperature level as smoothly as possible. This can be achieved by means of a regulating system, which comprises, for example, a computer, flow meters for fuel and combustion gases, and valves. The system may be designed in the following manner. When operating at superstoichiometric conditions, such a state occurs: the reduction in the amount of input combustion gas results in an increase in temperature. An allowable minimum stoichiometric ratio higher than 1.0 is also set. At sub-stoichiometric conditions, the state changes to: an increase in the amount of input combustion gas results in an increase in temperature, and the minimum stoichiometric ratio is replaced by a maximum stoichiometric ratio below 1.0. At the moment of the transition to the sub-stoichiometric operation, the regulating system is immediately given this new state, which means that the transition is obtained as fast as the valve(s) change position. The opposite state changes and limits apply when moving from sub-stoichiometric operation to over-stoichiometric operation.

From the above description, it should now be apparent that a method in accordance with at least one embodiment of the present invention is capable of achieving a change between gasification at higher loads (i.e., sub-stoichiometric conditions) and combustion at lower loads. The invention allows such a change not only during start-up of the cyclone burner, but also during its operation. Furthermore, unlike other prior art burners that can be operated at sub-stoichiometric conditions in one zone and over-stoichiometric conditions in another zone simultaneously, the present method makes it possible to utilize the same zone of a cyclone burner to shift between two different stoichiometric conditions.

It is also clear that the inventive idea described above enables an improved turndown ratio (the relation between the maximum and minimum possible loads to be burned in a cyclone burner). This may be useful, for example, when it is desired to change to the furnace (which is connected to the cyclone burner) output, typically in a local thermal power plant (up to 30-50 megawatts) or even in a domestic boiler (several hundred kilowatts). During operation, the temperature within the combustor may be kept relatively constant, however, the amount of fuel and thus the output may be different, depending on, for example, day or night operation. The improved turndown ratio of the swirl burner facilitates changing between requiring more or less output. In the prior art burners, it may be necessary to interrupt the operation of the burner at some time, because it is not possible to produce a sufficiently low output, and therefore the burner must be started again when a larger output is required again. The inventive idea of the present invention however provides a larger possible adjustment range.

Drawings

Fig. 1 is a schematic diagram showing the relationship between the stoichiometric ratio and the adiabatic temperature when wood pellets are used as fuel.

FIG. 2 is a schematic diagram showing the theoretical minimum particle velocity at the top of the combustor as a function of combustor diameter.

Fig. 3 is a schematic diagram showing the lower limit gas velocity calculated as a function of particle diameter and combustion chamber diameter.

Fig. 4 is another diagram showing the lower limit gas velocity calculated as a function of the particle diameter and the combustor diameter.

Fig. 5 is a schematic diagram showing the adjustment ratio depending on the stoichiometric ratio and the relative gas flow rate.

Fig. 6 is another schematic diagram showing the adjustment ratio.

Fig. 7 is a schematic diagram showing the turndown ratio in the case where recirculated exhaust gas is added to the combustion gas.

Fig. 8 is another schematic diagram showing the adjustment ratio in the case where recirculated exhaust gas is added to the combustion gas.

Fig. 9 is still another schematic diagram showing the adjustment ratio in the case where the recirculated exhaust gas is added to the combustion gas.

Fig. 10 is yet another schematic diagram showing the adjustment ratio in the case where recirculated exhaust gas is added to the combustion gas.

Fig. 11 shows the forces acting on the particles in a vertical cyclone burner.

Detailed Description

Fig. 1 is a schematic diagram showing the relationship between the stoichiometric ratio and the adiabatic temperature when wood pellets are used as fuel. The wood pellets may have a lower heating value (or lower heating value) of 18.2 MJ/Kg. The figure shows that for a stoichiometric ratio of approximately 0.95, the highest temperature can be obtained. If more oxygen is provided relative to the oxygen required for complete combustion of the fuel, i.e. over-stoichiometric conditions, the temperature will be lower. For example, a stoichiometric ratio of 2.0 results in an adiabatic temperature of 1200 ℃. Similarly, if less oxygen is provided so that more sub-stoichiometric conditions are achieved, the temperature will also be lower. For example, a stoichiometric ratio of 0.5 would result in a temperature of approximately 1400 ℃. As already explained above, it may be desirable to keep the temperature within a certain range in order to obtain a satisfactory operability. Thus, if operation in the range of 1100 ℃ to 1300 ℃ is desired for this particular fuel, the sub-stoichiometric and over-stoichiometric ratios will be maintained at approximately 0.37 to 0.45 and 1.8 to 2.25, respectively.

FIG. 2 is a schematic diagram showing the theoretical minimum particle velocity at the top of a horizontal swirl burner combustion chamber as a function of combustion chamber diameter. As already explained above, the lower limit gas flow rate is set according to the situation that the particles just at the uppermost position (top) of the combustion chamber are prevented from falling down. Tangential particle velocity (V) if it is assumed that the radial drag is negligible_p，t) Is that

V_{p, t} = \sqrt{gR} .

This is shown in fig. 2. For example, a combustion chamber with a diameter of 0.3m, 0.6m or 1.2m will result in a minimum particle velocity of 1.2m/s, 1.7m/s and 2.4m/s at the top, respectively.

FIG. 3 is a schematic diagram showing the lower limit gas velocity calculated as a function of particle diameter and combustor diameter in a horizontal cyclone burner. Tangential gas velocity (V_g，t) Must be higher than the minimum particle velocity (V)_p，t). As already explained above, the tangential gas velocity V_g，tShould be so high that the particle velocity at the upper position (phi 180 DEG) in the combustion chamber of the cyclone burner is higher than the calculated minimum particle velocity (V)_p，t). Taking this as a boundary condition, the gas velocity can be solved from the following differential equation:

it was found that the lower limit gas velocity (V) was obtained when the swirl burner combustion chamber radius and particle diameter were increased_g，t) Will increase. This is shown in fig. 3. In the figure, the horizontal axis represents the particle diameter in mm, and the vertical axis represents the lower limit gas velocity in m/s. Three curves are drawn, the bottom curve for a combustion chamber with a diameter of 0.3m, the middle curve for a combustion chamber with a diameter of 0.6m and the top curve for a combustion chamber with a diameter of 1.2 m. For these calculations, assume a friction factor of 0.5, a coefficient of drag of 0.44, and a gas density of 0.28Kg/m³And the particle density is 1000Kg/m³. The figure shows that for a particle diameter of, for example, 2.0mm (e.g., crushed wood pellets), the lower limit gas velocity is about 11 to 13m/s (depending on the size of the combustion chamber). For smaller particle diameters (such as crushed wood pellets), for example 0.5mm, the lower limit gas velocity is as low as 6 to 8 m/s.

When the fuel particles enter the combustion chamber of the cyclone burner, they will rapidly release their volatile substances. Thus, the particle density will also decrease. It is therefore appropriate to calculate the lower limit gas velocity after devolatilization based on the particle density. For wood particles, the density is generally 250kg/m³. This is shown in fig. 4. Thus, except that the density of the granules in FIG. 4 is 250kg/m³Instead of 1000kg/m³Except that all the input data are the same as those of the schematic shown in fig. 3. For a particle diameter of 0.5mm, a lower limit gas velocity of about 3 to 5m/s is sufficient to obtainThe minimum particle velocities calculated above for the different combustion chamber diameters (1.2m/s, 1.7m/s and 2.4m/s) were obtained. If the empirically derived upper limit gas velocity is about 30m/s, the turndown ratio will be about 30: 5, i.e. 6: 1, for a given combustion temperature and particle diameter of 0.5 mm. The turndown ratio can be further extended if it is allowed to vary the combustion temperature with load.

Fig. 5 is a schematic diagram showing the adjustment ratio depending on the stoichiometric ratio and the relative gas flow rate. In this example, it is assumed that the adiabatic temperature within the combustion chamber of the cyclone burner is approximately 1300 ℃. The horizontal axis represents the relative load factor of the swirl burner. The left vertical axis represents the stoichiometric ratio inside the combustion chamber. The right vertical axis represents the relative gas flow in the combustion chamber, i.e., the ratio between the actual gas flow and the minimum gas flow, or in most cases, the ratio between the actual gas velocity and the lower limit gas velocity.

Referring to the left side of the figure, when a relatively small amount of fuel (i.e., a small load) is fed into the combustion chamber, a relatively large amount of oxygen-containing combustion gas, such as air, is provided so that an over-stoichiometric condition exists within the combustion chamber. The stoichiometric ratio is maintained at about 1.8, as shown by the dashed line L1, in order to maintain a temperature of about 1300 ℃. As the load increases, by increasing the rate at which combustion gas is fed into the combustion chamber, the amount of combustion gas also increases, thereby maintaining an over-stoichiometric condition. This is shown by the inclined left part of the curve L2. In this case, the stoichiometric ratio was kept substantially constant at 1.8. The amount of load that will operate under over-stoichiometric conditions can be determined from the lower and upper gas velocities (typically 4 times the lower gas velocity). The limiting gas velocities are indicated by the horizontal straight lines L4 (lower limit) and L5 (upper limit) passing through the graph. Thus, when the load, and thus also the gas velocity, is increased from the relative load factor of 1 on the horizontal scale, the upper limit gas velocity will eventually be reached. This occurs at 4 on the horizontal scale. Thus, a cyclone burner operating at over-stoichiometric conditions would be limited to a 4: 1 turndown ratio.

After the upper gas velocity has been reached under over-stoichiometric conditions, a shift operation is performed in order to obtain sub-stoichiometric conditions, thereby allowing a further increase in load. The transition to sub-stoichiometric conditions is performed by decreasing the gas velocity before it reaches or exceeds the upper limit gas velocity, as indicated by line L6. In this case it coincides with a lower gas velocity at a sub-stoichiometric ratio of about 0.45 (at 4 on the horizontal scale) in order to maintain a temperature of about 1300 c. Now not with excess oxygen, but with a lack of oxygen. As indicated by dashed line L7, a sub-stoichiometric ratio of about 0.45 will be held substantially constant while allowing further increases in the amount of fuel delivered to the combustion chamber. The amount of fuel can be increased and therefore the gas flow is also increased, as shown by line L8, until such a load: an upper limit gas velocity is reached under the load. This occurs at 16 on the horizontal scale. This means that a turndown ratio of 16: 4, i.e. 4: 1, will be obtained if the cyclone burner is to be operated only at this sub-stoichiometric ratio. By combining the two operating modes with two stoichiometric conditions, a theoretical turndown ratio of 16: 1 can be achieved.

The above process is reversible. Thus, one can start from the right side of the curve in fig. 5, i.e. under sub-stoichiometric conditions. As the load decreases and thus the gas velocity also decreases, the lower limit gas velocity will eventually be reached. At this point, the over-stoichiometric condition is shifted by increasing the gas velocity. Thereafter, the load may be reduced even further until the gas velocity is reduced to a lower limit gas velocity in order to maintain a substantially constant over-stoichiometric ratio.

Fig. 6 is another schematic diagram showing the adjustment ratio. In this case, the same fuel is used in the same combustion chamber as in fig. 5. However, it is now desirable to have an adiabatic temperature of about 1100 ℃ within the combustion chamber. This temperature can be achieved for an over-stoichiometric ratio of about 2.2 and a sub-stoichiometric ratio of about 0.38. As can be seen from fig. 6, the transition from over-stoichiometric to sub-stoichiometric conditions at the upper limit gas velocity will result in the gas velocity being below the lower limit gas velocity, as indicated by the downward arrow. Similarly, as shown by the upward arrows, when there is a lower limit gas velocity, a transition from sub-stoichiometric conditions to over-stoichiometric conditions will result in a gas velocity that is much higher than the upper limit gas velocity. This means that when transitioning from one stoichiometric condition to another, the gas velocity will pass through the upper and/or lower limit gas velocities in order to maintain the desired temperature and in order to obtain overlap.

The difficulties shown in fig. 6 can be overcome by adding recirculated flue gas, with low or no oxygen content, to combustion gases, such as air, with high oxygen content.

Accordingly, fig. 7 is a schematic diagram showing the turndown ratio in the case where recirculated exhaust gas is added to the combustion gas. As in fig. 6, the desired temperature in the combustion chamber is 1100 deg.c. A fixed amount of recirculated flue gas (15% of the minimum gas flow) is mixed into the combustion gases before they are fed into the combustion chamber. The amount of recirculated exhaust gas is indicated by the straight horizontal dashed line L9 at the bottom of the figure. Straight lines corresponding to those in fig. 5 are denoted by the same reference numerals.

As shown in fig. 7, the minimum load at sub-stoichiometric conditions is further extended due to the application of the recirculated exhaust gas. The recirculated exhaust gas does not increase the heat released from the fuel while increasing the total gas flow. Thus, a minimum limit gas flow, i.e., a lower limit gas velocity, is reached at lower loads. In addition, the recirculated exhaust gas serves as a gas ballast. Additional combustion gases are therefore required in order to maintain the desired temperature. This further increases the total gas flow and reaches a minimum limit at further reduced loads. According to fig. 7, this limit is about 3.5 on the horizontal scale, instead of about 6 in fig. 6.

Under over-stoichiometric conditions, the added exhaust gas will partially replace the excess combustion gases. Thus, the total gas flow will remain the same compared to without any exhaust gas recirculation, but the stoichiometric ratio will vary between about 1.8 and 2.1 with changing load (see dashed line L1). The advantage is that as the load is reduced, the oxygen concentration will be reduced, resulting in less formation of nitrogen oxides. Thus, in fig. 7 and 6, the upper load limit of the over-stoichiometric condition is reached at 4 on the horizontal scale. Although there is no overlap in fig. 6, due to the spread in sub-stoichiometric conditions to the minimum load, an overlap and thus a possible transition region PTR is obtained in fig. 7. The possible transition region PTR is defined by a lower limit speed under sub-stoichiometric conditions and an upper limit speed under over-stoichiometric conditions. In contrast to having a "narrow" straight line L6 as shown in fig. 5, a wider possible transition region PTR is obtained in the case shown in fig. 7. This means that in the case shown in the figure it is not necessary to wait until the limit gas velocity is reached in order to make the transition to other stoichiometric conditions. Instead, the transfer may be performed at an earlier time when the fuel quantity is a fuel quantity that does not exceed the limits set by the other limit gas speeds of the other stoichiometric conditions. For example, when transitioning from sub-stoichiometric to over-stoichiometric conditions, the transition may be made at a load corresponding to 4 (upper limit, over-stoichiometric) on the horizontal scale of FIG. 7, or at the latest at a load corresponding to about 3.5 (lower limit, sub-stoichiometric) on the horizontal scale. It can be noted that according to FIG. 7, the adjustment ratio is 18: 1. However, because a given swirl burner has a maximum load capacity, i.e., an acceleration limit due to the devolatilized particles from the accelerated combustion, and because the gas velocity is proportional to the load, it is likely that the maximum load will be reached before the gas velocity reaches the upper limit gas velocity under sub-stoichiometric conditions. Thus, the maximum load capacity or acceleration limit indirectly determines the speed limit. However, the advantage is that the intervals (turndown ratios) in which it is possible to operate at sub-stoichiometric conditions are enlarged, which is preferable from an environmental point of view because less nitrogen oxides are formed. This is further illustrated in fig. 8.

Fig. 8 is another schematic diagram showing the adjustment ratio in the case where recirculated exhaust gas is added to the combustion gas. In this case the desired temperature is 1300 c and the graph is plotted for the same type of fuel in the same cyclone burner as in figure 5. However, fig. 8 shows the case where 15% of the recirculated exhaust gas is in the combustion gas. Comparing the two figures, since the minimum load under sub-stoichiometric conditions is further shifted to the left in fig. 8, it is clear that the possible transition zone is larger when using recirculated exhaust gas. Nevertheless, it is preferable to operate as much as possible under over-stoichiometric conditions, and if exhaust gas recirculation is not eliminated at higher loads, the use of exhaust gas may negatively impact the overall turndown ratio. In FIG. 8, for example, the overall turndown ratio is approximately 12.5: 1 instead of 16: 1 as in FIG. 5.

Fig. 9 and 10 show the effect of introducing a larger portion of the gas as recirculated exhaust gas. In these examples, the recirculated exhaust gas is 45% of the minimum gas flow, and the desired temperature is 1100 ℃ in fig. 9, and 1300 ℃ in fig. 10. It may be noted that this higher exhaust gas recirculation will result in a larger possible transition region. It is also noted that in FIG. 10, the operable range under sub-stoichiometric combustion is extended approximately to a relative load factor of 1.

FIG. 11 will be discussed below in order to derive the lower limit tangential gas velocity for a "vertical" cyclone burner (i.e., a burner comprising a central axis of symmetry extending vertically and a circular cross-section in a horizontal plane). In a corresponding manner in the case of horizontal cyclone burners, the limiting gas velocity is set by means of vertically falling particles.

It is assumed in the following that fuel particles are not carried out through the outlet of the combustion chamber. For reasons of simplicity, the air flow is described as a horizontal swirl (no vertical drag) and the radial air flow is considered negligible, which results in a balance of forces acting on the fuel particles 2 shown in fig. 11. The fuel particles abut against the inner wall 4 of the combustion chamber. To prevent the particles from falling down, by frictional forces F_fAnd centrifugal force F in the direction of the incline_cTo balance the gravity F_gThe angle of the plane to the horizontal plane H is α.

F_f+F_ccos(α)＝F_gsin(α)

Centrifugal force F_cAnd gravity F_gCan be expressed as:

F_{c} = m_{p} \frac{V_{p, t}^{2}}{R}

F_g＝m_pg

wherein m is_pIs the mass of the particles, V_p，tIs the particle tangential velocity, R is the radius of the combustion chamber of the cyclone burner, and g is the gravitational constant. According to the formula, friction force F_fAnd normal force F_NIn proportion:

F_f＝μF_N

F_N＝F_gcos(α)+F_csin(α)

where μ is the friction factor or coefficient of friction. This results in the following relationship:

F_f+F_ccos(α)＝F_gsin(α)

thus, the minimum tangential particle velocity will be:

from the above it is clear that if a) the radius R is decreased, b) the tangential particle velocity V is increased_p，tOr c) increasing the coefficient of friction, μ, a steeper slope may be possible.

To maintain the tangential particle velocity, a tangential drag force F_d，tThe friction force F must be balanced_f. The friction is equal in all directions.

Wherein C is_dIs the coefficient of traction, A_pIs the fuel particle cross-sectional area, p_gDensity of combustion gas and V_g，tTangential gas velocity.

The minimum tangential gas velocity will therefore be:

by particle density p_pMultiplying by particle volume to substitute mass m_p，d_pIs the particle diameter and rewrites the particle cross-sectional area A_p

To obtain

By substituting the minimum tangential particle velocity into the above expression, the following equation can be obtained:

the larger or heavier the particles, the larger the required combustor radius and the higher the required tangential gas velocity. In addition, as the angle α increases and the friction coefficient decreases, the lower limit gas velocity will increase.

Claims

1. A method of controlling a combustion process in a slagging-free, cyclonic burner after start-up thereof, the method comprising:

feeding fuel into a cylindrical combustion chamber of the cyclone burner;

feeding an oxygen-containing combustion gas into said combustion chamber at a tangential velocity defining a lower gas velocity and an upper gas velocity for said combustion gas;

maintaining the velocity of the combustion gases between the limit gas velocities;

maintaining one of the two stoichiometric conditions, the sub-stoichiometric condition and the over-stoichiometric condition, by controlling the amount of oxygen fed relative to the amount of fuel fed, i.e., the fuel load;

shifting to the other of the two stoichiometric conditions while preventing the combustion gases from attaining velocities outside the range defined by the lower and upper gas velocities.

2. The method of claim 1, further comprising:

maintaining a temperature within said combustion chamber within a temperature range of 700 ℃ -1300 ℃, preferably 900 ℃ -1100 ℃, wherein each temperature point within said temperature range together with said limit gas velocity defines a respective minimum fuel load and a respective maximum fuel load for shifting from one stoichiometric condition to the other stoichiometric condition of said two stoichiometric conditions.

3. The method of claim 2, further comprising:

mixing recirculated flue gas or other low oxygen-containing gas or inert gas with the oxygen-containing combustion gas prior to feeding the combustion gas into the combustion chamber, thereby reducing the minimum fuel load at sub-stoichiometric conditions.

4. The method of claim 2, further comprising:

recycled exhaust gas or other low oxygen-containing gas or inert gas is mixed with the oxygen-containing combustion gas prior to feeding the combustion gas into the combustion chamber, thereby reducing the oxygen concentration and thus the formation of nitrogen oxides under over-stoichiometric conditions at the same total gas flow rate.

5. The method of claim 1 or 2, wherein the act of maintaining stoichiometric conditions comprises maintaining a substantially constant stoichiometric ratio in order to control the temperature.

6. A method according to claim 2 or 3, wherein the stoichiometric ratio is maintained within defined limits while the temperature within the combustion chamber is controlled by the amount of recirculated flue gas or other low oxygen-containing gas or inert gas to be mixed with the oxygen-containing combustion gas.

7. A method as claimed in any one of claims 1 to 6, which includes feeding the fuel in the form of solid fuel particles, such as wood particles, preferably wood pellets, typically crushed wood pellets of maximum diameter 4 mm.

8. The method of claim 7, the method comprising:

controlling an amount of combustion gas for a relatively small amount of fuel fed into the combustion chamber so that over-stoichiometric conditions prevail within the combustion chamber;

increasing the amount of combustion gas by increasing the rate at which combustion gas is fed into the combustion chamber as the amount of fuel increases, thereby maintaining an over-stoichiometric condition;

before the gas velocity reaches the upper limit gas velocity or when the fuel amount is such that: sub-stoichiometric conditions may be achieved which meet the requirements for a temperature in the combustion chamber of 700-1300 c, preferably 900-1100 c and the gas velocity is equal to or above the lower limit gas velocity, the shift to sub-stoichiometric conditions being made by reducing the velocity of the combustion gas to reduce the relative amount of combustion gas.

9. The method of claim 8, wherein after transferring to sub-stoichiometric conditions, the method further comprises:

when the amount of fuel is further increased, the amount of combustion gas is increased by increasing the rate at which combustion gas is fed into the combustion chamber while maintaining sub-stoichiometric conditions.

10. The method of claim 7, the method comprising:

controlling an amount of combustion gas for a relatively large amount of fuel fed into the combustion chamber so that sub-stoichiometric conditions prevail within the combustion chamber;

reducing the amount of combustion gas by reducing the rate at which combustion gas is fed into the combustion chamber when the amount of fuel is reduced, thereby maintaining sub-stoichiometric conditions;

before the gas velocity reaches the lower limit gas velocity or when the fuel amount is such that: the over-stoichiometric condition can be achieved to meet the requirement that the temperature in the combustion chamber is 700-1300 c, preferably 900-1100 c and the gas velocity is equal to or lower than the upper limit gas velocity, shifting to the over-stoichiometric condition by increasing the velocity of the combustion gas to increase the relative amount of combustion gas.

11. The method of claim 10, wherein after transferring to an over-stoichiometric condition, the method further comprises:

when the amount of fuel is further reduced, the amount of combustion gas is reduced by reducing the rate at which combustion gas is fed into the combustion chamber while maintaining the over-stoichiometric condition.

12. A method according to any one of claims 7 to 11, wherein the lower limit gas velocity is the lowest velocity for maintaining at least the majority of the fuel particles circulating within the combustion chamber.

13. Method according to any of claims 7-12, wherein for a cyclone burner with a horizontally extending central symmetry axis of the combustion chamber, the tangential lower limit gas velocity V at the top of the combustion chamber can be calculated by solving the following differential equation_g，t：

Boundary conditions are satisfied for [ 180 ]

V_{p, t} = \sqrt{gR} .

Wherein

Mu-friction factor

C_dCoefficient of traction

A_pCross-sectional area of fuel particle

ρ_gDensity of combustion gas

O is at an angle to the vertical, i.e. 180 DEG at the top of the combustion chamber

V_g，tTangential gas velocity

V_p，tTangential particle velocity

m_pMass of particles

g-gravity constant

Radius of combustion chamber of swirl burner

S-distance traveled by the particle along the circumference

14. Method according to any of claims 7-12, wherein for a cyclone burner having a combustion chamber with a vertically extending central symmetry axis, the tangential lower limit gas velocity V can be calculated by solving the following equation_g，t：

Wherein,

V_g，ttangential gas velocity

g-gravity constant

Radius of combustion chamber of swirl burner

Alpha is the angle to the horizontal

Mu-friction factor

d_pDiameter of fuel particles

ρ_pDensity of fuel particles

ρ_gDensity of combustion gas

C_dCoefficient of traction

15. A method according to any of claims 7-14, wherein the upper limit gas velocity is the highest velocity allowed for preventing a large amount of unburnt particles from leaving the combustion chamber, said velocity being 20-50m/s, preferably 25-40m/s, such as about 30 m/s.